Threshold-like transcriptomic responses of European beech to simulated future climates

Beech (Fagus sylvatica) is one of the most important forest tree species in Europe, both in terms of land coverage and ecosystem services. Although it is projected to be strongly affected by climate change, insights into its functional genomic responses to realistic future scenarios are still lacking. We conducted a fully controlled ecotron experiment to examine the transcriptomic responses of 125 young, naturally regenerated beech trees exposed to regionalized dynamic climate series representing a reference period (1987-2016) and two future climate scenarios (Representative Concentration Pathways, RCP2.6 and RCP8.5, for 2071-2100) in Germany. RCPs describe alternative greenhouse gas trajectories established by the IPCC, ranging from strong mitigation (RCP2.6) to high, unmitigated emissions (RCP8.5), the latter representing a worst-case future scenario. Although both scenarios produced substantial transcriptional changes relative to the reference period, RCP8.5 elicited markedly stronger responses than RCP2.6, with nearly twice as many differentially expressed genes and unique transcriptional programs across functional categories that were not predictable from the RCP2.6 transcriptomes. Our study reveals emergent functional responses in European beech under projected late 21st-century climate conditions in Germany and highlights potential biomarkers for monitoring ecosystem responses to climate change.